Protein tyrosine phosphatases (PTPases) are key signaling molecules and potential targets for developing inhibitors as novel therapeutics. Our preliminary studies demonstrate for the first time that sodium stibogluconate (SS) and glucantime (GT), antimony (Sb) based anti-leishmania drugs of unknown mechanism, are inhibitors of selective PTPases with activity against cancer cells in vitro and in mouse models at nontoxic doses and in synergy with IFNs. We propose the following specific aims: Aim 1. To test the hypothesis that SS anti-cancer activity functions via inactivating PRLs and SHPs, we will: a), assess the role of PRLs in SS activity against cancer cell lines using SS-insensitive PRL mutants; b), assess the role of SHPs in SS synergy with IFNa in cell growth inhibition using SHP-deficient cell lines and SHP transfectants; c), define the pharmacodynamics of SS in inhibiting PRLs/SHPs in mouse models; d), determine the effects of SS on PRLs-regulated signaling molecules in cancer cells; e), determine the effects of SS on IFNa signaling molecules in WM9 melanoma cells against which SS and IFNa showed a synergistic effect. Aim 2. To test the hypothesis that SS inactivates target PTPases via covalent modification of a catalytic cysteine residue in the enzymes through the Sb in the drugs while the gluconic acid conjugated to Sb facilitates Sb/cysteine interactions and defines the drugs' PTPase specificity, we will: a), define the nature and significance of SS-induced mass increase in PRL-2 by mass spectrometry and mutational analysis; b), assess the ability of SS to modify MKP1 and PRL-1R86 mutant insensitive to SS inhibition; c), determine the effects of Sb and gluconic acid on PTPases sensitive or insensitive to SS inhibition; d), determine the PTPase inhibitory activity and anti-cancer activity of different molecular weight fractions of SS, which is mixture of polymers of Sb/gluconic acid complexes of 100-4000 Da. Aim 3. To test the hypothesis that GT has anti-cancer activity via targeting PTPases, we will: a), determine its inhibitory activity against a panel of PTPases; b), define its inhibitory mechanism by mass spectrometry and mutational analysis; c), assess its anti-cancer activity against human cancer cell lines in vitro and in mouse models; d), assess the role of its target PTPases in mediating its anti-cancer activity.